The present invention relates to a DC--DC converter especially of the type in which a semiconductor active element such as a transistor is used instead of a flywheel or catching diode which provides a path for a current when a main switching transistor is turned off.
The efficiency of the DC--DC converters is much influenced not only by the characteristics of main switching transistors but also by the characteristics of flywheel diodes as will be described in detail below. The forward voltage drop of the flywheel diodes greatly influences the efficiency especially when the output voltages of the DC--DC converters are low. A recovery time and a reverse recovery time of the transient characteristics influence the efficiency. More specifically, when a forward recovery time and a reverse recovery time become comparable with respect to the operating or on-off frequency of a main switching transistor, the losses caused during the recovery time adversely affects the efficiency. In the DC--DC converters, a current or voltage applied across a flywheel diode abruptly rises and falls so that the losses due to the transient characteristics give rise to a serious problem especially when the switching frequency is high. Therefore, the Schottky barrier type diodes or fast-recovery type diodes have been used as the flywheel diodes, but the diodes with a reverse recovery time of less than 50 ns are not available so that it is difficult to attain a higher degree of efficiency at higher frequencies. The Schottky barrier type diodes are advantageous over the fast-recovery type diodes in that the forward voltage drop is small and the turn-off time is faster, but their breakdown voltage is low so that they can be used only at low voltages. The forward voltage drops of these diodes are of the order of 0.5 V at the maximum current ratings and will exceed 0.25 V and 0.2 V, respectively, even when they are used at a tenth and a hundredth of the maximum current ratings. Therefore, with the Schottky barrier type diodes and a fast-recovery type diodes, the attainable characteristics and efficiency of the DC--DC converters are limited. Furthermore, the maximum switching frequency is also limited.
In the phase detectors (phase discriminators, phase sensitive amplifiers, lock-in amplifiers) which are used in communication equipment and measuring instruments, semiconductor active elements such as FETs, bipolar transistors, etc. have been long used as rectifying elements so that it may be considered that such semiconductor active elements can substitute the flywheel diodes in the DC--DC converters. However, so far no attempt has been successful. The transistors have many advantageous features such as a low forward resistance, a high reverse resistance, excellent transient characteristics so that if they could be used instead of the conventional flywheel diodes, the DC--DC converters which can operate at higher frequencies with a higher degree of efficiency could be provided. However, in practice, it is extremely difficult to attain the correct synchronization between the on-off operations of a main switching transistor and a transistor which is used as a flywheel diode. Mainly because of the manufacturing tolerances, the characteristics of the main switching transistors and flywheel transistors vary from one another. As a result, a flywheel transistor (that is, the transistor used instead of a flywheel diode) is turned on before a main switching transistor is turned on, so that an excessive reverse current flows into the flywheel transistor. Even when the on-off operation of the flywheel transistor can be correctly synchronized with that of the main switching transistor, an excess reverse current flows into the flywheel transistor due to the difference in instantaneous voltage between the main switching transistor and the flywheel transistor in the transient periods so that the flywheel transistor is damaged or degraded. When the flywheel transistor is turned on after the main switching transistor has been turned off, the drop in efficiency inevitably results. Thus, it has been extremely difficult in practice to use the semiconductor active elements as the flywheel diodes in order to operate the DC--DC converters at higher frequencies with a higher degree of efficiency.